Method of melting materials



April 2, 1963 R. H. MILLER 3,084,032

METHOD OF' MELTING MATERIALS Filed June 16, 1959 ZZ Z @i ZJM Wlan/0M Pfl/:75 9

United States Patent O 3,084,032 METHOD OF MELTING MATERIALS Richard H. Miller, Chicago, lll., assignor to Astravac Corporation, Burlingame, Calif., a corporation of California Filed June 16, 1959, Ser. No. 820,766 Claims. (Cl. 23-308) The present invention relates generally to materialmelting methods and more particularly to an improved method of melting bodies of material having a high electron emission at its melting point, such as tungsten, tantalum, and carbon.

Bodies of material such as tungsten, tantalum, carbon, etc., have previously been melted in electron gun furnaces, wherein the kinetic energy of electrons moving at high velocity is transferred to a target formed of the material to be melted. Ordinarily, such electron gun furnaces have included an electron gun which focuses emitted electrons on the material to be melted. The material is maintained at a high positive voltage relative to the `filament in the electron gun in order to accelerate the electrons.

The maximum current of the electron gun beam is limited by beam instability resulting from the space charge associated with large current density beams. The current of the beam can be increased by increasing the supply voltage but there is a practical limit to such an increase because the tendency to spark increases rapidly with increasing supply voltage.

Thus, the capacity of single gun electron furnaces is limited to the melting of small bodies of material.

The capacity of the electron gun furnaces has been increased `somewhat by increasing the number of electron guns in the furnace. However, space limitations place a maximum limit on the power obtainable.

Besides the power limitations, the electron gun furnaces have excessive radiation losses, and the electron guns in such devices have short lives because during operation the electron gun or guns are bombarded by positive ions.

lAn object of the present invention is the provision of an improved method of using the energy of electrons to melt bodies of material having a high electron emission at its melting point, such as tungsten, carbon, etc. Another object is the provision of a relatively eflicient method of melting material having a high electron emission at its melting point.

Various other objects and advantages of the present invention will become apparent by reference to the following description and accompanying drawings.

In the drawings:

FIGURE 1 is a diagrammatic view of a melting apparatus for carrying out the method of the present invention;

FIGURE 2 is a partial diagrammatic view of another embodiment of a melting -apparatus for carrying out melting of materials in accordance with the present invention;

FIGURE 3 is a partial diagrammatic View of still another embodiment of a melting -apparatus for carrying out melting of materials in accordance with the present invention; and

FIGURE 4 is a vertical, cross-sectional, schematic View of a melting apparatus of the type shown in FIG- URE l.

The melting of materials in accordance with the present invention is preferably carried out by apparatus comprising a gas-tight chamber which is evacuated by a pumping means. Means are provided in the chamber for supporting at least two bodies of a material having 3,084,032 Patented Apr. 2, 1963 ICC a high electron emission at its melting point, the bodies being `supported in spaced-apart relationship. Means are j provided in the chamber for preheating the -bodies to emission temperature. The melting apparatus also includes an alternating current power source connected to the bodies, the power source being of suticient voltage to cause electron bombardment of those sides of the bodies which face each other with an accompanying melting of those sides.

More specifi-cally, the melting apparatus shown in the drawings is particularly adapted to melt tungsten in accordance with the method of the present invention. However, other materials having a high electron emission at their melting points, such as tantalum, carbon, etc., may also be melted in the melting apparatus in accordance with the method of the present invention.

For purposes of explanation, the invention will first be described with reference to FIGURE l wherein two horizontally extending, spaced-apart, longitudinally aligned rods 10 and 12 of material are disposed in a gastight chamber 14. The chamber 14 is evacuated by a suitable vacuum pump i6.

Material such as tungsten, when it is heated near its melting point, is capable of emitting thousands of amperes of electron current per square inch. The emitted electrons, if there is no electric eld, will collect in the general vicinity -of the emitting surface and produce a space charge which will limit the number of electrons being given off by the surface. By setting up a suitable electric iield in the vicinity of the emitting surface, the electrons are accelerated away from the surface by the iield, the energy acquired by the electrons `depending upon the strength of the tield. When the electrons strike or bombard a surface, they give up their kinetic energy to the surface, and if the surface is at a high enough temperature, the added energy is sufiicient to melt the surface.

If two Ispaced-apart bodies of material having a high electron emission at its melting point are preheated in a suitable vacuum to their emission temperature and a suitable alternating voltage is applied to the bodies, the bodies not only maintain their emission temperature but also are melted by the electron bombardment.

As shown in FIGURE 1, the rods 10' and 12 are preheating, the emission temperature for tungsten, for exam-l ple, being about 2800= K. (4580 F.). Helical coils 18 and 2t) are disposed concentrically about the rods 10 and 12 respectively, and the coils 18 and Ztl are connected in series to a suitable source 22 of high frequency alternating current. pgtwmtayhinlvhich thelodimgy be preheated is by borrrbardingihigdsaathlectnognsjr'm heated by resistive heating.

By connecting a suitable alternating current power source 24 to the rods, as shown diagrammatically in FIG- URE l, on the positive half cycles of the voltage, the electrons emitted by the instantaneously negative rod will be accelerated to the instantaneously positive rod, giving up their heat to the positive rod. On the negative half cycles, the electron ow would be reversed. By reversing the direction of current flow frequently enough, the rods are prevented from cooling below their emission temperature. Any frequency over a few cycles per second is satisfactory and preferably, for convenience, a standard power line 60-cycle current may be used.

The intensity of the electron flow between the rods is limited by the space charge, that is, there are so many electrons in the beam that the electric accelerating eld mll another way in which the rods I rraybe'r'i-r'liated to their emission temperature is byrr" initially touching the rods together and then passing ailhigh current through the rods, the rods thereby being heated to their emission temperature by induction-typ] at the instantaneously negative rod is annulled. Arcing occurs between the rods when the impurities in the rods, vaporized by the electron bombardment, enter the stream of electrons, and are positively ionized thereby. The positive ions drift toward the instantaneous negative rod and neutralize the space charge, thus resulting in a large increase in electron current with an accompanying arc.

Such arcing is minimized by maintaining the chamber 14 under a sucient vacuum so that the vapor or gas released by the rods 1t) and 12 is quickly exhausted. Preferably the vacuum pump 16 is of sufficient capacity t0 maintain the chamber under a vacuum which is less than the vapor pressure of the released gases or vapor. A vacuum of l5 mm. of mercury or better is generally preferred, with the capacity of the pump depending upon the rate of gas evolution from the melted surfaces.

While most of the gas is exhausted by a high vacuum, it should be realized that it is impractical to instantaneously remove all of the gas from between the rods and 12. Therefore, a certain amount of ionization occurs between the rods which results in some neutralization of the space charge and some arcing. Heating may be accomplished under this condition, but in order to avoid arcing, suitable stabilizing means or current limiters (not shown), such as reactors, are connected to the power source 24.

Preferably, for the most efficient heating, the chamber 14 is essentially evacuated of all gases, however heating may be accomplished with an inert atmosphere, such as helium, neon, etc. in the heating chamber at a pressure of approximately lO-3 mm. After the inert gas is ionized by the electron flow, the instantaneously negative rod is heated by the ions transferring their energy thereto and the instantaneously positive rod is heated by electron ow. After the gas ionizes, the operating current increases rapidly. In order to limit the operating current to a level which will not damage the power source, suitable current limiters should be provided in the power source.

Radiation loss from rods of material melted in the above described manner is greatly reduced over previously available electron furnaces since the melting surface faces another surface at melting temperature instead of cooler walls.

Another feature of the invention is that the spacing between different areas of the surfaces which face each other tends to remain uniform. A high spot on a surface will increase the electric field at that spot and thus will increase the electron flow to the high spot with an accompanying increase in heating. Thus, the high spot melts faster than the low spots.

As the surfaces of the rods which face each other melt, the spacing between the rods becomes greater, thus decreasing the electron current ow between the rods. Therefore, a suitable means (not shown in FIGURE l) is provided to move the rods 10 and 12 toward each other to compensate for the melting.

The material melted by the electron bombardment is collected in a suitable crucible 26 located beneaththe melting zone of the rods 10 and 12. Some of the drops of melted material may tend to adhere to the rods 10` and 12. When this happens, suitable means (not shown in FIGURE l), such as a vibrator which vibrates the rods at a suitable frequency may be provided in the chamber 14 to assure that the drops fall off the rods 10 and 12.

'Ihe material in the crucible 26 may be maintained in a melted condition by electron bombardment from a generally vertically extending bar 28 of the material spaced above the melted material in the crucible 26. A suitable source 30 of alternating current is connected between the bar 28 and the melted material in the crucible 26 to accelerate the electrons. Of course, the crucible 26 may be replaced by a second vertically extending bar (not shown) of larger size than the first mentioned vertically extending bar 28, the melted material collecting on the larger bar.

Melted material within the crucible 26 may be stirred by causing the path of the electron current extending between the bar 28 and the melted material to be changed to different positions on the surface of the melted material. The current path may be changed through the interaction of the elecetron current with an alternating magnetic field which is suitably set up to extend generally transversely of the path of current flow. In order to prevent a cut-olf of current flow and thus a discontinuation in the stirring force when the alternating voltage between the bar 28 and the crucible 26 is near its minimum, the magnetic field is set up so that it alternates at the same frequency and at approximately the same phase as the alternating voltage applied between the bar 28 and the crucible 26. Of course, if a discontinuous stirring force can be tolerated, a static or direct current magnetic field may be set up.

Polyphase power systems may also be used to operate the electron furnace. For example, FIGURE 2 shows diagrammatically how three-phase power may be utilized. In this embodiment, three rods 32, 34 and 36 of the material to be melted are extended outwardly in the general form of a Y, that is7 approximately at equal angular intervals. The inner ends of the rods 32, y34 and 36 are suitably spaced from each other and, after the rods 32, 34 and 36 are pre-heated in a manner such as that described above, a suitable source 38 of three-phase power is connected to the rods 32, 34 and 36.

When the power is applied to the rods 32, 34 and 36, the electron flow is from one bar to another around the system. The material melted is collected in a crucible 40 situated beneath the melting zone of the rods. As those surfaces of the rods 32, 34 and 36 which face one another melt, the rods are advanced toward one another by a suitable means (not shown).

The three-phase power may also be used, as in the arrangement shown in FIGURE 3, to keep the material in a crucible 42 in a melted state. In this case, a pair of rods 44 and 46 of the material to be melted extend upwardly at an angle to the surface of the melted material in the crucible 42. A suitable source 48 of three-phase power is connected to the rods 44 and 46 and to the melted material in the crucible 42.

A construction of an electron furnace of the type shown in FIGURE 1 is shown schematically in FIGURE 4, the furnace including a gas-tight chamber 50` which cornprises a tubular housing 52 having its lower end closed, and a circular cover '54 fitting on an upper flange 56 of the housing 52. A vacuum seal is assured by an O-ring 58 disposed between the flange and the cover. The cover 54 is fastened to the flange 56 by suitable means, such as screws 60.

After the cover 54 is sealed on the housing 52, the chamber 50 is evacuated through a pair of vertically disposed, relatively large pipes 62 at the lower closure wall 64 of the housing 52 by a suitable vacuum pump (not shown) which is of such capacity that a high vacuum is maintained in the chamber 50.

A pair of spaced apart rods 66 and `68, which are to be melted, are supported in horizontally extending, longitudinally aligned relationship by a pair of vertically extending clamps 70 and 72, respectively, of conductive material. The clamps 70 and 72 are supported and moved horizontally relative to each other by a horizontally extending threaded shaft 74 of non-conductive material which is engaged with threaded apertures 76 and 78 in the clamps 70 and 72, respectively. One half of the shaft 74 is provided with a right-hand thread and the other half is provided with a left-hand thread so that when the shaft 74 is rotated the clamps 70 and 72 move relative to each other. The shaft 74 is supported in bearings `80 suitably mounted to the housing 52.

The clamps 70 and 72 are prevented from turning on the shaft 74 by a horizontally extending guide bar 82 of non-conductive material located adjacent the cover 54, the bar 82 being slidably engaged by slots 84 and 86 in the upper ends of the clamps 70 and 72, respectively. A vibrator 88 is connected to the guide bar 82 to vibrate the clamps 70 and 72 at a suitable frequency, which in turn vibrate the rods 66 and 68 thereby assuring that the drops of melted material fall olf the rods 66 and 68. Driving power for the shaft 74 is provided by a motor 90 connected through a magnetic clutch 92 to one end of the shaft 74. The rate of feed of the rods 66 and 68 toward each other is governed by the rate at which the rods 66 and 68 melt. The rate of feed of the rods 66 and 68 may be governed manually or by automatic equipment which may be controlled by the amount of current passing through the rods 66 and 68.

Electrical conductors 94 and 96 are respectively connected between the clamps 70 and 72 and high voltage terminals 98 and 100 on the cover 54. A suitable source (not shown) of A.C. power is connected to the high voltage terminals 98 and 100. Suitable arc protection may be included in the power source to prevent arcing between the rods 66 and 68 from damaging the power source.

In the illustrated embodiment, the rods 66 and 68 are preheated to emission temperature by helical coils 102 and 104 disposed concentrically about the rods 66 and 68, respectively, the coils being of sufficient diameter so that there is no contact between the coils and the rods. The coils 102 and 1104 are supported by support members 106 and 108 of non-conductive material connected respectively to the coils 102 and 104. The coils 102 and 104 are connected through conductors 110 and 112, respectively, to a suitable source (not shown) lof RF power, the conductors 110 and 112 being connected to the ends of the coils 102 and 104 and extending through suitable vacuum seals 113 in the housing 52.

In order to avoid excessive electron heating and mechanical loading of the coils 102 and 104 by material evaporated from the rods 66 and 68 during electron bombardment, the coils, after they are de-energized, are retracted away from the melting zone (i.e., the zone between the rods). Also, cooling water may be circulated through the coils 102 and 104 for further cooling.

As shown in FIGURE 4, the coils 102 Iand 104 are moved along the rods 66 and 68 by a horizontally extending threaded shaft 114 which is engaged with threaded apertures 116 and 118 in the support members 106 and 108, respectively. The shaft 114 is journaled in bearings 119 suitably mounted to the housing S2. One half of the shaft 114 is provided with a right-hand thread and the other half is provided with a left-hand thread so that when the shaft 114 is rotated, the support members 106 and 108 move relative to each other.

The support members 106 and 108 are prevented from turning on the shaft 114 by a horizontally extending guide bar 120 mounted to the closure wall 64 of the housing 52, the bar 120 being slidably engaged by slots 122 and 124 in the lower ends of the support members 106 and 108, respectively.

Driving power for the shaft 114 is provided by suitable means, such as a motor (not shown) acting through a magnetic clutch 126 connected to one end of the shaft 114. The motor may be energized manually or automatically so that the coils are adjacent the opposed ends of the rods 66 and 68 when the coils 102 and 104 are energized, and are moved away from the melting zone when the coils 102 and 104 are de-energized.

As shown in FIGURE 4, the melted material from the rods l66 and 68 is collected in a cuplike Crucible 128 supported on the lower wall 64 of the housing 52.

In operation, the rods 66 and 68 which Iare to be melted are mounted in spaced apart relationship in the clamps 70 and 72. The cover 54 is then sealed on the housing 52 and the chamber 50 is evacuated. The coils 102 and 104 are moved toward each other until they are adjacent the opposed ends of the rods and then the RF power source is switched on. When the rods 66 and 68 reach their emission temperature, the A.C. power source is switched on and the RF power source is switched off. The coils 102 and 104 are then moved away from the melting zone, and the rods 66 and 68 are advanced toward each other as the opposed ends thereof melt.

The amount of heating applied to the rods depends upon the applied voltage land the spacing between the rods. For example, `for rods whose section dimensions are large compared to the spacing between the rods, 10 kilovolts peak alternating applied volt-age will produce heating of 5.2 kilowatts per square inch at `a l inch spacing between rods. The heating capability increases rapidly as the spacing decreases, reaching 21.5 kilowatts per square inch at 1/2 inch spacing for the same voltage. Thus a pair of 3 inch diameter rods (approximately 7 square inches in area) will heat at 300 kilowatts at 1/2 inch spacing and l0 kilovolts peak applied alternating voltage. Of this, kilowatts is delivered to each rod for heating purposes. Should the spacing increase to 1 inch, the power will decrease to 73 kilowatts from 300 kilowatts. Ordinarily, the electron furnace is operated somewhere in this range of power.

Various changes and modifications may be m'ade in the above described method without departing from the spirit or scope of the invention. Various features of the invention are set forth in the accompanying claims.

I claim:

1. A method of melting material having a high electron emission at the melting point, said method comprising supporting at least two spaced apart bodies of said material in a gas-tight chamber, promptly minimizing ionizable gases in the region between said bodies by establishing a high vacuum in said region of less than 10-3 mm. Hg and less than the vapor pressure of gases released from said bodies, whereby a space charge is maintained between said bodies and arcing therebetween is minimized, glrlwgsaid bodies to emission temperature, applying a ternating voltage to the bodies of suflicient yamplitude to produce electron bombardment of those sides of said bodies which face each other with an accompanying melting of said sides, and limiting the current flow through said bodies to less than the current caused by sustained arcing.

2. A method of melting material having a high electron emission at the melting point, said method comprising supporting at least two spaced apart bodies of said material in a gas-tight chamber, promptly minimizing ionizable gases in the region between said bodies by establishing a high vacuum in said region of less than 10-3 mm. Hg and less than the vapor pressure of gases released from said bodies, whereby a space charge is m-aintained betiveen said bodies and arcing therebetween is minimized, pre eatin said bodies to emission temperature, a l in marini voltage to the bodies of suicient amliliilud to produce electron bombardment of those sides of said bodies which face each other with an accompanying melting of said sides, limiting the current ow through said bodies to less than the current caused by sustained arcing, and moving said bodies toward each other as said sides melt.

3:. A method of melting material having a high electron emission at the melting point, said method comprising supporting a pair of horizontally extending, spaced apart, longitudinally aligned rods in `a gas-tight chamber, promptly minimizing ionizable gases in the region between said bodies by establishing a high vacuum in said region of less than 10*3 mm. Hg and less than the vapor pressure of gases released from said bodies whereby a space charge is maintained between said bodies and arcing therebetween is minimized, preheating the opposed faces of said rods to emission temperature, applying alternating voltage to said rods of suflicient amplitude to produce electron bombardment of those sides of said rods which face each other with an accompanying melting of said sides, moving said rods toward each other as said sides of -said rods melt, limiting the current flow through said bodies to less than the current caused by sustained arcing and collecting the melted material.

4. A method of melting tungsten material having a high electron emission at the melting point, said method comprising supporting at least two spaced -apart bodies 'of said tungsten material in a gas-tight chamber, promptly minimizing ionizable gases in the region between said bodies by establishing a high vacuum in said region of less than 10-3 mm. Hg and less than the vapor pressure of gases released from said bodies whereby a space charge is maintained between said bodies and arcing therebetween is minimized, preheating said bodies to emission temperature, applying high alternating voltage to the bodies of sulicient amplitude to produce electron bombardment of those sides of said bodies which face each other with an accompanying melting of said sides, and limiting the current flow through said bodies to less than the current caused by sustained arcing.

5. A method of melting tungsten material having a high electron emission at the melting point, said method comprising supporting at least two spaced -apart bodies of said tungsten material in a gas-tight chamber, promptly minimizing ionizable gases in the region between said bodies by establishing a high vacuum in said region of at least 105 mm. Hg and less than the vapor pressure of gases released from said bodies whereby -a space charge is maintained between said bodies and arcing therebetween is minimized, preheating said bodies to emission temperature, applying high alternating voltage to the bodies of sufficient -amplitude to produce electron bombardment of those sides of said bodies which face each other with an accompanying melting of said side-s, and limiting the current ow through said bodies to less than the current caused by sustained arcing.

References Cited in the le of this patent UNITED STATES PATENTS 1,133,508 Schoop Mar. 30, 1915 2,189,387 Wissler Feb. 6, 1940 2,737,566 Wuppermann Mar. 6, 1956 2,795,819 Lezberg et al June 18, 1957 Y 2,818,461 Gruber et al. Dec. 31, 1957 2,897,539 McMillan Aug. 4, 1959 

1. A METHOD OF MELTING MATERIAL HAVING A HIGH ELECTRON EMISSION AT THE MELTING POINT, SAID METHOD COMPRISING SUPPORTING AT LEAST TWO SPACED APART BODIES OF SAID MATERIAL IN A GAS-TIGHT CHAMBER, PROMPTLY MINIMIZING IONIZALE GASES IN THE REGION BETWEEN SAID BODIES BY ESTABLISHING A HIGH VACUUM IN SAID REGION OF LESS THAN 10-3 MM. HG AND LESS THAN THE VAPOR PRESSURE OF GASES RELEASED FROM SAID BODIES, WHEREBY A SPACE CHARGE IS MAINTAINED BETWEEN SAID BODIES AND ARCING THEREBETWEEN IS MINIMIZED PREHEATING SAID BODIES TO EMISSION TEMPERATURE, APPLYING ALTERNATING VOLTAGE TO THE BODIES OF SUFFICIENT AMPLITUDE TO PRODUCE ELECTRON BOMBARDMENT OF THOSE SIDES OF SAID BODIES WHICH FACE EACH OTHER WITH AN ACCOMPANYING MELTING OF SAID SIDES, AND LIMITING THE CURRENT FLOW THROUGH SAID BODIES TO LESS THAN THE CURRENT CAUSED BY SUSTAINED ARCING. 